HAARP is a favorite target for conspiracy theorists. It's an easy target because the science behind it is rather difficult to understand and even harder to explain to people with no background in science. So I'm starting this thread to:

A) Gather useful resources that can help explain what HAARP actually does and how it does it
B) Explain why HAARP can NOT do the various things that are claimed (change the weather, create earthquakes, mind control).

First a very brief attempt at an understandable explanation of what HAARP does:

About 60 miles up the atmosphere is very thin. The radiation from the sun here is so powerful it knocks electrons off the air molecules creating an ionized layer called the ionosphere. HAARP is square grid of antenna that transmit radio waves straight up. The radio waves are absorbed by the electrons in the ionosphere. This heats them up slightly (but a lot less than the normal heat from the sun). HAARP does not affect the 60 miles of atmosphere below the ionosphere. Scientists do experiments by heating a small patch of the ionosphere in various ways and observing the results. They do this mostly because variations in the ionosphere affect radio communication, and they want to study this so they can improve radio communication.​

(obviously still not very understandable of most non-scientific people)

The first places to start is the official HAARP web site and FAQ. Here I've bolded the key numbers

What Is HAARP?
The High-frequency Active Auroral Research Program (HAARP) is the world’s most capable high-power, high frequency (HF) transmitter for study of the ionosphere. The principal instrument is the Ionospheric Research Instrument (IRI), a phased array of 180 HF crossed-dipole antennas spread across 33 acres and capable of radiating 3.6 megawatts into the upper atmosphere and ionosphere. Transmit frequencies are selectable in the range of 2.7 to 10 MHz, and since the antennas form a sophisticated phased array, the transmitted beam can take many shapes, can be scanned over a wide angular range and multiple beams can be formed. The facility uses 30 transmitter shelters, each with six pairs of 10 kilowatt transmitters, to achieve the 3.6 MW transmit power.

What Is HAARP Used For?

The goal of the research at HAARP is to conduct fundamental study of the physical processes at work in the very highest portions of our atmosphere, called the thermosphere and ionosphere. This research falls into two categories (1) active, which requires the use of the Ionospheric Research Instrument and (2) passive, which only uses monitoring instruments.

The ionosphere starts at about 60 to 80 km altitude and extends up above 500 km altitude. There are free electrons and ions in the ionosphere that radio waves can interact with. HAARP radio waves heat the electrons and create small perturbations that are similar to the kinds of interactions that happen in nature. Natural phenomena are random and are often difficult to observe. With HAARP, scientists can control when and where the perturbations occur so they can measure their effects. In addition, they can repeat experiments to confirm the measurements really show what researchers think they do.

During active ionospheric research, the signal generated by the transmitter system is delivered to the antenna array, transmitted in an upward direction, and is partially absorbed, at an altitude between 100 to 350 km (depending on operating frequency), in a small volume a few hundred meters thick and a few tens of kilometers in diameter over the site. The intensity of the HF signal in the ionosphere is less than 3 microwatts per cm2, tens of thousands of times less than the Sun's natural electromagnetic radiation reaching the earth and hundreds of times less than even the normal random variations in intensity of the Sun's natural ultraviolet (UV) energy which creates the ionosphere. The small effects that are produced, however, can be observed with the sensitive scientific instruments installed at the HAARP facility and these observations can provide new information about the dynamics of plasmas and new insight into the processes of solar-terrestrial interactions.

The effects of this added energy are limited to a small region directly over the HAARP observatory ranging in size from 9 km in radius to as much as 40 km in radius.
It is important to realize that HAARP interacts only with charged (or ionized) particles in a limited region of the ionosphere directly over the facility. Interaction occurs because a charged particle (electron or positive ion) will react to an external electric field. HAARP does not interact with the neutral atoms and molecules that make up the bulk of the gas at all atmospheric heights. Neutral (non-ionized) particles, which outnumber ionized particles by 500:1 or greater, remain unaffected.

Effects produced by HAARP are thermal in nature and do not result in new ionization. HAARP is not able to produce artificial ionization for the following two reasons.

The frequencies used by the HAARP facility are in the High Frequency (HF) portion of the spectrum. Electromagnetic radiation in the HF frequency range is non-ionizing - as opposed to the sun's ultraviolet and X-ray radiation whose photons have sufficient energy to be ionizing.

The intensity of the radiation from the completed HAARP facility at ionospheric heights will be too weak to produce artificial ionization through particle interactions. The power density produced by the completed facility will not exceed 3 to 4 microwatts per cm2, about two orders of magnitude below the level required for that process.

Basically, the IRI is what is known as a phased array transmitter. It is designed to transmit a narrow beam of high power radio signals in the 2.8 to 10 MHz frequency range. Its antenna is built on a gravel pad having dimensions of 1000' x 1200' (about 33 acres). There are 180 towers, 72' in height mounted on thermopiles spaced 80' apart in a 12 x 15 rectangular grid. Each tower supports near its top, two pairs of crossed dipole antennas, one for the low band (2.8 to 8.3 MHz), the other for the high band (7 to 10 MHz). The antenna system is surrounded by an exclusion fence to prevent possible damage to the antenna towers or harm to large animals. An elevated ground screen, attached to the towers at the 15' level, acts as a reflector for the antenna array while allowing vehicular access underneath to 30 environmentally-controlled transmitter shelters spaced throughout the array. Each shelter contains 6 pairs of 10 kW transmitters, for a total of 6 x 30 x 2 x 10 kW = 3600 kW available for transmission. The transmitters can be switched to drive either the low or high band antennas. Electric prime power is provided from an on-site power plant housing five, 2500 kW generators, each driven by a 3600 hp diesel engine. Four generators are required for operation of the IRI and the fifth is held as a spare. From a control room within the Operations Center, the transmission from each of the 180 crossed-dipole antennas is adjusted in a precise manner under computer control. In this manner, the complete array of antennas forms a narrow antenna pattern pointed upward toward the ionosphere. The transmitted signal diverges (spreads out) as it travels upward and is partially absorbed, at an altitude which depends on the transmitted HF frequency, in a small volume several tens of miles in diameter and a few hundred meters thick directly over the facility. The remainder of the transmitted signal either reflects back toward the earth or passes through the ionosphere into space, continuing to diverge as it does so. By the time it reaches the ionosphere, the intensity of the HF signal is less than 3 microwatts (0.000003 watt) per cm2, thousands of times less than the Sun's natural electromagnetic radiation reaching the earth and hundreds of times less, even, than thevariations in intensity of the Sun's natural ultraviolet (UV) energy which creates the ionosphere.

The IRI would transmit radio waves over the frequency range 2.8 to 10 MHz. The transmitted radio wave beam would occupy a conical volume roughly 30 miles in diameter at an altitude of 300 miles. The transmitted radio waves would have up to 3.3 MW of power, only slightly higher than waves transmitted by radio and television stations.

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The last bit is rather an exaggeration, as the most powerful radio station ever, Radio XERA, was less than 1.0 MW (far in excess of what is allowed now for radio transmission), and TV towers only get into the hundreds of kilowatts.

These seem mostly meant to emphasize the safety but don't give much insight imo. It's what they would say to a group of concerned citizens. But what would they say to a fellow scientist or a visiting general?

the transmitted signal diverges (spreads out) as it travels upward and is partially absorbed, at an altitude which depends on the transmitted HF frequency, in a small volume several tens of miles in diameter and a few hundred meters thick directly over the facility. The remainder of the transmitted signal either reflects back toward the earth or passes through the ionosphere into space, continuing to diverge as it does so. By the time it reaches the ionosphere, the intensity of the HF signal is less than 3 microwatts (0.000003 watt) per cm2, thousands of times less than the Sun's natural electromagnetic radiation reaching the earth and hundreds of times less, even, than the variations in intensity of the Sun's natural ultraviolet (UV) energy which creates the ionosphere.

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The energy of the sun's electromagnetic radiation is spread over the whole spectrum, most of which will not be absorbed by the ionosphere.

Specific heat of air is about 1 J/g.K, air density at 100 km is 10-10 g/cm3 so a volume 300 meters thick would contain 3.10-6 g/cm2 air. 3 microwatts per cm2 would heat this volume at a rate of 1°K per second, if all energy was absorbed. (Mean temperature at that height is about 200 K btw). So you could describe HAARP as heating hundred cubic kilometers of air at a rate of 1°K per second. (ballpark figure and likely too high, I simplified calculations, specific heat depends upon conditions, and assumed 100% absorption.)

While the sun's output can vary significantly, those variations are usually temporal, not spatial. We're not talking about a region with large variations (like 201°K ± 13.4°K) where HAARP would cause a small change ( below the standard deviation) in the mean value, in most cases HAARP will cause temperature differences in excess of the naturally occurring variation (at that time and place). That is exactly the aim and purpose of ionospheric modification experiments.

It is important to realize that HAARP interacts only with charged (or ionized) particles in a limited region of the ionosphere directly over the facility. Interaction occurs because a charged particle (electron or positive ion) will react to an external electric field. HAARP does not interact with the neutral atoms and molecules that make up the bulk of the gas at all atmospheric heights. Neutral (non-ionized) particles, which outnumber ionized particles by 500:1 or greater, remain unaffected.

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I'm not sure why they emphasize the neutral to charged particles ratio of 500:1 or greater. At 100 km altitude, it's much more, in the order of ten or hundred billion if I'm not mistaken, and they don't remain unaffected, at that height you can't heat up the ionized particles without heating the rest, they collide millions of times per second, so the added energy is distributed over all the particles. At higher altitudes, like 400 km you could get close to the 500 ratio, and mean free path length greatly increases so in a way the other particles are "less" affected, but I still don't see the relevance.

The transmitted radio waves would have up to 3.3 MW of power, only slightly higher than waves transmitted by radio and television stations.

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That's simply not true, not for TPO and certainly not for ERP. Maximum allowed ERP for FM radio in the US is 100 kW, HAARP can reach 5.1 GW. If you consider transmitter power output, the difference is still significant, the TPO of an FM station would be 10 to 20 kW, compared to the 3.3 MW of HAARP.
(the server at www.haarp.alaska.edu is taking too long to respond for the last two days btw.)

These seem mostly meant to emphasize the safety but don't give much insight imo. It's what they would say to a group of concerned citizens. But what would they say to a fellow scientist or a visiting general?

The energy of the sun's electromagnetic radiation is spread over the whole spectrum, most of which will not be absorbed by the ionosphere.

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But they refer specifically to the UV radiation, which is absorbed by the ionosphere.

Specific heat of air is about 1 J/g.K, air density at 100 km is 10-10 g/cm3 so a volume 300 meters thick would contain 3.10-6 g/cm2 air. 3 microwatts per cm2 would heat this volume at a rate of 1°K per second, if all energy was absorbed. (Mean temperature at that height is about 200 K btw). So you could describe HAARP as heating hundred cubic kilometers of air at a rate of 1°K per second. (ballpark figure and likely too high, I simplified calculations, specific heat depends upon conditions, and assumed 100% absorption.)

While the sun's output can vary significantly, those variations are usually temporal, not spatial. We're not talking about a region with large variations (like 201°K ± 13.4°K) where HAARP would cause a small change ( below the standard deviation) in the mean value, in most cases HAARP will cause temperature differences in excess of the naturally occurring variation (at that time and place). That is exactly the aim and purpose of ionospheric modification experiments.

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The biggest variation of course is the transition from day to night, where the variation is both spatial and temporal, and an vastly greater scale than HAARP can operate at.

I'm not sure why they emphasize the neutral to charged particles ratio of 500:1 or greater. At 100 km altitude, it's much more, in the order of ten or hundred billion if I'm not mistaken, and they don't remain unaffected, at that height you can't heat up the ionized particles without heating the rest, they collide millions of times per second, so the added energy is distributed over all the particles. At higher altitudes, like 400 km you could get close to the 500 ratio, and mean free path length greatly increases so in a way the other particles are "less" affected, but I still don't see the relevance.

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Perhaps you could supply so reference for the ten billion figure?

That's simply not true, not for TPO and certainly not for ERP. Maximum allowed ERP for FM radio in the US is 100 kW, HAARP can reach 5.1 GW. If you consider transmitter power output, the difference is still significant, the TPO of an FM station would be 10 to 20 kW, compared to the 3.3 MW of HAARP.

(obviously still not very understandable of most non-scientific people)

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Those People are also mostly uncommon with the altitudes of the atmosphere...

By refering to the Power of the sun, you should add Polar Ligths (Auoroa). This is High-Energy from the sun, acting in the Ionosphere. Exactly the same HAARP was disignet to do. To understand, what "60 Miles Altitude" and "ionosphere" means, the best way is to look at an Aurora from the side.

Pictures of Polar Ligths taken from the International SPace Station (ISS) show this the best. Like this one:

Maybe you should add a timelapse-Video from the ISS istead of a picture, they show the dynamics of Auroras very impressiv.

That´s the reason, why HAARP is named "High Frequency Active Auroral Research Program.

They want to know more about Auroras - by trying to make some kind of it. And they have reached the goal an published this success in March 2013. They have made a smal, ball-shaped artificial Aurora with HAARP in November 2012.

U.S. Naval Research Laboratory (NRL) research physicists and engineers from the Plasma Physics Division, working at the High-frequency Active Auroral Research Program (HAARP) transmitter facility, Gakona, Alaska, successfully produced a sustained high density plasma cloud in Earth's upper atmosphere.

"Previous artificial plasma density clouds have lifetimes of only ten minutes or less," said Paul Bernhardt, Ph.D., NRL Space Use and Plasma Section. "This higher density plasma 'ball' was sustained over one hour by the HAARP transmissions and was extinguished only after termination of the HAARP radio beam."

Artificial Ionospheric Plasma Clouds Sequence of images of the glow plasma discharge produced with transmissions at the third electron gyro harmonic using the HAARP HF transmitter, Gakona, Alaska. The third harmonic artificial glow plasma clouds were obtained with HAARP using transmissions at 4.34 megahertz (MHz). The resonant frequency yielded green line (557.7 nanometer emission) with HF on November 12, 2012, between the times of 02:26:15 to 02:26:45 GMT.
(Photo: SRI International—Elizabeth Kendall)

These glow discharges in the upper atmosphere were generated as a part of the Defense Advanced Research Projects Agency (DARPA) sponsored Basic Research on Ionospheric Characteristics and Effects (BRIOCHE) campaign to explore ionospheric phenomena and its impact on communications and space weather.

Using the 3.6-megawatt high-frequency (HF) HAARP transmitter, the plasma clouds, or balls of plasma, are being studied for use as artificial mirrors at altitudes 50 kilometers below the natural ionosphere and are to be used for reflection of HF radar and communications signals.

Past attempts to produce electron density enhancements have yielded densities of 4 x 105 electrons per cubic centimeter (cm3) using HF radio transmissions near the second, third, and fourth harmonics of the electron cyclotron frequency. This frequency near 1.44 MHz is the rate that electrons gyrate around the Earth's magnetic field.

The NRL group succeeded in producing artificial plasma clouds with densities exceeding 9 x 105 electrons cm3 using HAARP transmission at the sixth harmonic of the electron cyclotron frequency.

Optical images of the artificial plasma balls show that they are turbulent with dynamically changing density structures. Electrostatic waves generated by the HAARP radio transmissions are thought to be responsible for accelerating electrons to high enough energy to produce the glow discharge in the neutral atmosphere approaching altitudes of nearly 170 kilometers.

The artificial plasma clouds are detected with HF radio soundings and backscatter, ultrahigh frequency (UHF) radar backscatter, and optical imaging systems. Ground measurements of stimulated electromagnetic emissions provide evidence of the strength and frequency for the electrostatic waves that accelerated ambient electrons to ionizing velocities.

The NRL team is working with collaborators at SRI International, University of Alaska Fairbanks, University of Florida, and BAE Systems on this project to synthesize the observations with parametric interactions theory to develop a comprehensive theory of the plasma cloud generation. The next HAARP campaign, scheduled for early 2013, will include experiments to develop denser, more stable ionization clouds.

Well spotted, seems I made an error in my off the cuff calculation, lost a factor of 1000 somewhere ,maybe mixed up cubic cm with cubic dm or used higher estimates for air density at that height, I couldn't find exact values.
But it seems that at the Kármán line (100 km) the air density is about 1/2200000 the density at sea level. (http://en.wikipedia.org/wiki/Kármán_line)
Molar volume at standard pressure and temperature is about 22 liters, Avogadro's constant is 6 * 10^23 , or about 2.7 * 10^25 molecules per cubic meter at sea level, which gives a density at 100 km in the order of 10^19
Graphs for electron density in the E-layer show a value between 10^11 and 10^12 at 100 km during the day, 10^9 and 10^10 during the night (http://roma2.rm.ingv.it/en/research_areas/4/ionosphere), so ten to hundred million would be a more realistic day-time figure.

too late to edit my post it seems.. the heating calculation of the layer is valid at a height of around 113 km, not 100 km.

I was corresponding with one of the scientists who worked on HAARP, and asked him about how the energy is concentrated at a particular altitude. This was his response:

I've attached an entire doctoral thesis on the subject you brought up....(not mythesis, but one I was closely involved with). Well worth reading... in themeantime...here's a real short version.

The ionosphere has a continuous density gradient....partially due to the naturalpressure differential between the Earth's surface and outer space...but also dueto the electron density profile. This is the relative amount of free electronsreleased during the sun's ionization. The peak of free electron density occursat around 250 kilometers during the daytime.

The CRITICAL frequency and critical height, are largely dependent on the electrondensity profile. You can't just arbitrarily pick which altitude you want toheat...you have to coordinate it with the conditions which already exist. HAARP,as you pointed out....is incapable of creating a single ion...it has to work withwhat's there already. By tweaking the frequency slightly, you can select over asmall altitude range, which electron layer you're going to "wobble" the most.

More importantly though....the POLARAZATION you choose had a much more profoundeffect...this is why the antennas are circularly polarized. Using clockwisecircular polarization, you can head the lower level layers, while using thecounterclockwise mode, you can choose the higher altitude electrons. This isdescribed in the "Appleton-Hartree dispersion relationship"...probably the mostcomplex math used in all ionospheric science....spelled out early in Jackie'sthesis.

So...that's the reader's digest version. I do encourage you to read the wholethesis...tons of great info there!

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The thesis was publicly available on the internet, so I've attached it here. I've only dipped into it, but it's very interesting, and quite accessibly written.

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Molar volume at standard pressure and temperature is about 22 liters, Avogadro's constant is 6 * 10^23 , or about 2.7 * 10^25 molecules per cubic meter at sea level, which gives a density at 100 km in the order of 10^19
Graphs for electron density in the E-layer show a value between 10^11 and 10^12 at 100 km during the day, 10^9 and 10^10 during the night (http://roma2.rm.ingv.it/en/research_areas/4/ionosphere), so ten to hundred million would be a more realistic day-time figure.

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You are talking about electron density (electrons per m^3), which is a different thing to the neutral to charged particles ratio (the "500:1 or greater" ratio you were quibbling with).

From the thesis I just posted,

During daytime, the ratio of charged particles to neutral particles concentration can vary from 10e−8 at 100 km to 10e−4 at 300 km and 10e−1 at 1000 km altitude.

You are talking about electron density (electrons per m^3), which is a different thing to the neutral to charged particles ratio (the "500:1 or greater" ratio you were quibbling with).

From the thesis I just posted,

During daytime, the ratio of charged particles to neutral particles concentration can vary from 10e−8 at 100 km to 10e−4 at 300 km and 10e−1 at 1000 km altitude.

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That's why I gave both the electron density (10^11 to 10^12 per cubic meter) and the air density (10^19 molecules per cubic meter) at 100 km, my conclusion "so ten to hundred million would be a more realistic day-time figure" was the ratio between the two. I could have stated it more clearly, I agree.

As to the "quibbling", I agree it's a trivial aspect, why post that meaningless figure in the first place?

That's why I gave both the electron density (10^11 to 10^12 per cubic meter) and the air density (10^19 molecules per cubic meter) at 100 km, my conclusion "so ten to hundred million would be a more realistic day-time figure" was the ratio between the two. I could have stated it more clearly, I agree.

As to the "quibbling", I agree it's a trivial aspect, why post that meaningless figure in the first place?

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In context:

It is important to realize that HAARP interacts only with charged (or ionized) particles in a limited region of the ionosphere directly over the facility. Interaction occurs because a charged particle (electron or positive ion) will react to an external electric field. HAARP does not interact with the neutral atoms and molecules that make up the bulk of the gas at all atmospheric heights. Neutral (non-ionized) particles, which outnumber ionized particles by 500:1 or greater, remain unaffected.

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They are just making the point that most air will not be affected. The 500:1 is likely the ratio at the upper end of the altitudes at which HAARP is targeted. "or greater" refers to the air below that. It would make more sense phrased as "or vastly greater", and explained that the lower atmosphere has a very low proportion of charged particles, which is on reason why it does not heat the lower atmosphere.